1. Field of the Invention
The present invention relates to a developer composition for electrostatic latent images employed in electrophotography, electrostatic recording, electrostatic printing, etc. More specifically, the present invention relates to a two-component developer composition utilized in magnetic brush development showing high performance even under low-temperature, low-humidity conditions and high-temperature, high-humidity conditions.
2. Discussion of the Related Art
The chargeability of the toners usable in development of electrostatic latent images in electrophotography, etc. is an important factor for giving good image quality. This chargeability is variable by temperature and humidity conditions, and hence is susceptible to undergo environmental changes, the chargeability of the toners being increased under low-temperature, low-humidity conditions, and decreased under high-temperature, high-humidity conditions, thereby resulting in the deterioration of the image quality.
The binder resins usable in the toners for development of electrostatic latent images generally include various resins, such as homopolymers or copolymers of styrenic monomers, such as polystyrene, styrene-butadiene copolymers, and styrene-(meth)acrylic acid ester copolymers; homopolymers or copolymers of ethylenic monomers, such as polyethylenes and ethylene-vinyl acetate copolymers; polyester resins; epoxy resins; and polyamide resins. However, the chargeability of the toners is liable to undergo substantial changes by the environmental conditions, though they may have some differences depending on the kinds of the binder resins used. Particularly, when the resulting toners are used under high-temperature, high-humidity conditions, the deterioration of the formed images is liable to take place. Particularly in a case where a polyester resin is used as a binder resin, it has been pointed out that since the polyester resin is obtainable by condensation polymerization of an alcohol and a carboxylic acid, a large number of carboxyl groups, which are hydrophilic groups, are present in the resin. Therefore, hydrogen bonds are formed between the carboxyl groups and water molecules, which result in the lowering of the chargeability of the toners, and thereby the deterioration of the formed images under high-temperature, high-humidity conditions is liable to take place.
However, in the field of electrophotography, the polyesters resins are widely used as binder resins for toners owing to their excellent fixing ability and high negative chargeability. Particularly in cases for producing color toners, the superiority of the polyester resins is well supported when compared with other resins in transparency as well as in the fixing ability and the negative chargeability. From the above facts, the development of a developer having little influence to environmental changes, while sufficiently utilizing the properties inherently owned by the polyester resins, is in demand.
Recently, in view of the above, various proposals have been made in the preparation of polyester resin-based toners, including methods for adjusting acid values of polyester resins (Japanese Patent Laid-Open Nos. 55-40407 and 56-65147); methods for adjusting a total amount of acid values and hydroxyl values of polyester resins (Japanese Patent Laid-Open No. 3-163564); methods for substituting a nitrogen-containing functional group for a carboxyl group of polyester resins; methods for increasing hydrophobicity of polyester resin-based toners by subjecting the toners with low acid values and low hydroxyl values to a surface treatment.
Among the methods mentioned above, the methods for controlling the acid values of the polyester resins to a range lower than those of conventional polyester resins, for instance from 0 to 10 KOH mg/g, are known to sufficiently prevent the lowering of the chargeability particularly under high-temperature, high-humidity conditions, so that the deterioration of the formed images can be prevented.
On the other hand, the electrophotograhic techniques recently have a tendency of digitization. For instance, by employing image-treatment techniques or A.I. techniques, digitized copy machines and color digitized copy machines having high reliability document-producing or image-producing functions have been appeared. In these digitized methods, since the toning of the formed images is controlled by dotted areal toning utilizing dotted images, the image densities of the dotted images have to be maintained at high levels even when developed at a low development voltage region. Therefore, the toners usable for digitized methods are required to have distinctive properties, i.e. the properties of providing high image densities even at a low development voltage, which are different from those usable for image-forming processes utilizing analog optical methods whose image toning changes depending upon the levels of the development voltage.
However, in the toners comprising polyester resins having low acid values mentioned above, the resulting formed images normally can have only low image densities at a low development voltage, and hence their image densities at low development voltages must be further increased in order to use these toners with such excellent environmental properties for digitized methods.